I've heard of people who use the carcass to make soup. My SIL told me she does (how I don't know, since she eats at my house every year, so I don't know where she gets this turkey carcass). But the essential ingredient for soup is the gelatin in the bones. If the carcass is already baked, how does it make soup??

I use the neck of the turkey to make broth, but I don't use it for soup. I use it to make sure that I have tons and tons of gravy.